Eastern Ukraine was the support base for Russia-friendly President Viktor Yanukovych, who was ousted in February after months of protests in the capital, Kiev.

The demonstrations were ignited by his decision to back away from closer relations with the European Union and turn toward Russia.

Opponents of the government that replaced him alleged the new authorities will repress eastern Ukraine's large Russian-speaking population.

The vehicles stopped near the city administration building and flew Russian flags while residents chanted "Good job! Good job!"

One of the men who came in the vehicles, who identified himself only as Andrei, said the unit was part of Ukraine's 25th Brigade of Airborne Forces and that they have switched to the side of the pro-Russian forces. The Ukrainian Defence Ministry declined immediate comment.

The government in Kiev has struggled to reign in the rebels, as some security forces have switched over to the side of the insurgents. In the regional capital of Donetsk, armed militias seized the mayor's office.

Kiev authorities and Western countries say tens of thousands of Russian troops are stationed near the border with Ukraine and fear that unrest in the east will be used by Moscow as a pretext for a military incursion.